The present invention relates to a plunger mechanism used in machines for forming hollow glass articles (known as I.S. machines), whose function is to press the gobs of molten glass inside the blank mould.
The prior art includes a variety of these devices capable of working in Single Gob, Double Gob, Triple Gob and Quadruple Gob operation, all operated by pneumatic cylinders controlled by an electronic programmer via 3-way 2-position valves.
The recent plunger mechanisms used in all I.S. machines (see documents Emhart GB-B-2,058,042B, WO 94/22776, EP-A-761,611 and EP-A-802,167, document Heye WO 94/27922) take the form of double-acting pneumatic cylinders controlled by solenoid distributing valves under the control of an electronic programmer.
There also exists a document Owens EP-A-789,004 in which the linear motion of the plunger is provided by a motor with a hollow shaft in which there operates a roller screw which engages on the female thread on the inside of the hollow shaft. Control of this device is therefore electrical with motion being converted from rotary to linear by means of a roller screw and with position sensing by a resolver.
The present-day technology of pneumatic plunger mechanisms works extremely well in Blow-and-Blow production processes in which the blank is first formed by pneumatic compression of the glass inside the blank mould (to form the neck) and then by blowing into the glass through the plunger mechanism (to form the blank). The function of the plunger mechanism in this case is purely that of keeping the plunger in position while the neck is being formed (the raised or xe2x80x9csettle blowxe2x80x9d position) and of forming the blank (the lowered or xe2x80x9ccounter blowxe2x80x9d position), executing a maximum stroke of about 40 mm.
In Press-and-Blow and Narrow Neck Press-and-Blow production processes, the function of the plunger mechanism is to form the blank completely by pushing into the glass which has been poured into the blank mould. During this process the stroke of the plunger varies from 165 mm to 210 mm and during the whole of the compression stroke and part of the down stroke the plunger is in direct contact with the glass in the plastic state.
Control of the end-of-pressing position and of the up and down velocities of the plunger become extremely important in the Wide Neck Press-and-Blow (jars) and Narrow Neck Press-and-Blow (bottles) processes.
In the plunger mechanism the end-of-pressing position is determined by the amount of molten glass poured into the blank mould and therefore by the weight of the gob cut off. A negative variation of weight results in a longer plunger stroke while a positive variation of weight results in a shorter plunger stroke; this because the stroke of the plunger determines the filling volume of the blank mould. Sensing the variation of the plunger stroke determines the variation of the weight of the gob and hence the correction to be applied to the gob forming mechanism. In conventional plunger mechanisms this sensing is possible only by fitting a linear sensor inside the cylinder to monitor the plunger stroke (see document Heye WO 94/27922), whereas in standard mechanisms this is impossible.
Another problem in pneumatically controlled mechanisms is determining the velocity of displacement of the plunger which can cause tears on the surface of the glass where it contacts the plunger, thus reducing the mechanical strength of the finished container.
The control of velocity in the cylinders of present-day plunger mechanisms is normally by means of a needle valve which regulates the flow of compressed air during cylinder discharge. This type of regulation creates a nonlinearity of the pressures upstream and downstream of the piston, resulting in poor control of the velocity of displacement.
It is known that the movement of a pneumatic cylinder is difficult to control and for this reason a number of manufacturers use electronically controlled proportional valves capable of controlling the pressures, inside the cylinder, to follow a curve defined through a PC or PLC (document Emhart WO 94/22776).
The use of a motor with a hollow shaft and a Satellite-roller screw (document Owens EP-A-789,004) has the advantage of introducing electronic control over the stroke of the plunger but has the disadvantage that motion has to be converted from rotary to linear by a satellite-roller screw. This gives decidedly poor transmission efficiency, and a twisting torque is transmitted to the rod of the punch which must be counteracted by a system of linear guides.
Another disadvantage with using a servo-guided roller screw is the wear over time of the linear guide which compromises the correct alignment of the plunger with the blank mould, which is vitally important in press-and-blow processes.
The object of the invention is to provide a plunger mechanism that eliminates the mechanical problems described above and allows process control which is currently not possible with pneumatic cylinders.
According to the invention this object is achieved through the features defined in claim 1. Additional secondary features are set forth in claims 2-6.
The use of an electric servomotor allows continuous monitoring of the velocity of displacement of the plunger throughout its stroke. Moreover, by monitoring the torque of the motor during the final pressing stroke it is possible to monitor the pressing pressure and leave the position free in such a way that by means of the linear position transducer built into the motor, it is possible to verify the end-of-pressing position and therefore calculate in real time the variation in the weight of the gob of molten glass. The position signal, after processing, can be used to operate the gob weight sensing device present in the gob forming mechanism (usually known as the feeder).
The use of an electric servomotor also allows the velocity of displacement of the plunger to be monitored by means of the programming of electronic cams modifiable directly by the operator of the forming machine. It also introduces a movement which is constant and repeatable in time, unlike compressed air-operated mechanisms where the properties of the compressible gas do not guarantee that the movements will be constant over time.
The idea on which the invention is based is in practice to eliminate the pneumatic cylinder without using devices that can convert a motion from rotary to linear, by directly using a linear-type electric control device, for which a brushless linear servomotor of tubular type is suitable. The tubular-type linear motor utilizes the principle of the linear motor with the difference that the windings and magnets are unrolled linearly into a toroidal shape rendering the motor similar to a pneumatic cylinder, instead of being unrolled linearly into a plane.
The advantage of employing a linear servomotor of wound-rotor type in the plunger mechanism is that the operation of the motor shaft is precisely balanced, suffering no radial thrust relative to the axis of the motor.
The attractive force generated by the toroidal magnets of the motor (moving part) on the toroidal windings of the motor (static part) are balanced without introducing any force perpendicular to the axis of the motor shaft.
In practice the tubular-type linear motor has the same functionality as a pneumatic cylinder but with all the known advantages of electrical drive with electronic control of the displacement.
The axial force developed by the linear motor will have a single linear component directed along the axis of the motor; this will eliminate the perpendicular components typical of flat linear motors and the twisting components typical of recirculating ball screws or satellite-roller screws.
The tubular-type linear motor, which will find application in carrying out the present patent, must be a motor designed for this specific application because the dimensions of the finished mechanism must be extremely small in order to allow it to be housed inside the structure of the machine module where it is currently used (see the standard linear motor produced by Indramat-Mannesman).
The present invention therefore provides a plunger device comprising a height-adjustable lower support for receiving the tubular-type linear motor which controls the up and down movements of the pressing plunger. The said mechanism may be constructed with a single motor for Single Gob processes, with two motors for D.G. processes, with three motors for T.G. processes and with four motors for Q.G. processes.
The device allows compressed air cooling of the plunger, as in the pneumatic mechanism, using air fed through the lower plate of the motor. Fixed to the lower plate of the motor is a cooling tube which runs through the interior of the hollow shaft of the linear motor.
To prevent cooling air from being discharged inside the motor there are dynamic seals between the cooling tube and the motor shaft.
Cooling air is discharged through the upper cylinder of the plunger.
Monitoring of the switching of currents and of the motor torque is either by means of Hall sensors mounted on the windings or by means of an analogue linear transducer mounted inside the motor. The analogue linear transducer is of the coil type and is formed on the cooling tube. The reading of the variation of inductance between tube and motor shaft, during the relative movement, provides the position signal which is used both to control the motor and as a feedback signal for gob weight control. In the gob weight control the position feedback is used to operate the drive control board which operates the motor for adjusting the height of the rotating tube (gob weight adjustment system present in the gob forming mechanisms).